The application of coatings on various substrates by an electrostatic coating process is well known. For example, many automotive finishes are applied by such a method. The composition applied by this process is an electrically chargeable powder mixture of a film forming polymer containing a pigment or pigments of the desired color, as well as other processing additives. The preparation of such powder coating compositions for application by powder coating spraying applicators has presented a problem in the past with platy pigments.
Pigments which are composed of platelets rely on their lamellar structure for their maximum appearance effect. Examples of such platelets include metallic flakes such as aluminum flakes, bronze flakes, and stainless steel flakes and natural or synthetic pearlescent pigments exemplified by, for instance, natural pearlescence or a metal oxide-coated mica such as titanium dioxide-coated mica, reduced titanium dioxide-coated mica and iron oxide-coated mica, and titanium and/or iron oxide coated glass flakes, iron oxide coated aluminum flakes, metal-oxide coated silica flake, alumina flake and graphite flake, bismuth oxychloride, and optically variable pigments. In the course of the manufacture of powder coating compositions containing such metallic or pearlescent pigments, the lamellar structure is destroyed during the extrusion or grinding processes which are used to incorporate non-platy materials into the resin system. As a result, the resulting finish exhibits a reduced luster effect.
In order to avoid the foregoing problem, the industry has dry blended the pigments and polymer powder, i.e., the powder carrier is mechanically blended with the pigments. Unfortunately, the pigment and powder coating particles have significantly dissimilar surface energies and particle sizes and therefore they do not mix readily or remain well dispersed during spray-out. In addition, the pigments are normally metal oxides, which always retain a hydrous film that drastically inhibits surface charge retention.
A bonding process was developed to overcome the problems encountered as a result of the dry blending process. In the bonding process, the powder base and pigment are mixed together and then heated to a temperature sufficient to soften the surface of the powder particles so that the resin particles can adhere to the surface of the pigment particles. The process works reasonably well for corona electrostatic charging applications. Unfortunately, it still leaves two distinctly different surfaces remaining in the final mixture, namely the powder surface and the surface of the laminar pigment that are now agglomerated to some degree. It does not work well for tribo electrostatic charging systems. This process also requires additional processing steps to prepare the powder coating composition and also it must be especially adapted to each selected combination of pigment and powder base.
U.S. Pat. No. 5,378,275 discloses a mica pigment-containing water base paint composition which is the combination of a water soluble or water dispersable film forming polymer and a mica-containing pigment. The mica pigment can be mica flakes or metal oxide-coated mica flakes which have been uniformly coated with a melamine resin by slurring and then volatilizing the organic solvent present.
U.S. Pat. Nos. 4,116,628 and 4,205,997 disclose powdery pearlescent pigment compositions which are coated with a solid polymer in order to protect the pigment prior to use.
A method of coating inorganic and organic particles such as pigment particles and colorant particles with a plastic dispersion is disclosed in U.S. Pat. No. 5,268,197.
A powder coating composition is disclosed in U.S. Pat. No. 3,986,007 in which synthetic resins, hardeners and pigments such as metals, metal alloys and mica for coatings with a metallic appearance are prepared by a process in which the individual components are mixed in a finely divided state under an atmosphere of a protective gas and the mixture is plasticized with homogenization at a temperature at which the viscosity of the synthetic resin is higher than the drop viscosity.
An improvement over the foregoing prior art is set forth in U.S. Pat. No. 5,824,144. A process is described in which the metal platelets are provided with a viscous surface layer of polymer or "sticky" liquid material. When this treated pigment is blended with powder coating compositions, the powder agglomerates to the surface of the pigment thereby minimizing color separation. The coating results in a better charge on those coated pigment particles which do not become attached to the powder materials, i.e., they accept a charge which is closer to the charge on the powder coating particle surfaces than the uncoated pigment particles. While this process represents an advance over the art, further improvements are desired.
An improved method and product has now been discovered. Advantageously, it is not limited to pigments but is applicable to any laminar platelet. In addition, the method allows the surface energy of the product to be significantly lowered in a controlled manner so that the pigment exhibits a hydrophobic surface that is now compatible with the formulation. This hydrophobic surface facilitates use of this product in many other applications in which pearlescent pigments are employed. Examples include, but are not limited to, incorporation into plastic articles, e.g., making masterbatches, "long-wear" and oil based cosmetic formulations and weather resistant exterior coatings.
It is the object of the present invention to provide a new process for preparing platelets having hydrophobic surface characteristics making them more suitable for incorporation into various formulations, in particular powder coating compositions suitable for corona and tribo electrostatic applications and formulations requiring hydrophobic surface properties. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.